Contact materials for vacuum switches



United States Patent U.S. Cl. 75-170 12 Claims ABSTRACT OF THEDISCLOSURE Added to nickel substantially free from oxygen, nitrogen andhydrogen is bismuth or tellurium in an amount of 1 to 10% by weight toprovide a contact material for use in vacuum switch excellent inanti-welding property without any increase in cutting through current.Also, in order to improve the anti-welding property of nonmagneticcontact materials for use in vacuum switches, 0.05 to 5% by weight ofbismuth or tellurium and 25 to 50% by weight of copper are added tonickel substantially free from oxygen, nitrogen and hydrogen. Thestarting material nickel is refined through its vacuum melt with carbon.As a result of such refining treatment the refined nickel and hence thecontact material may include a small' amount of carbon for example, 1 to2.2% by weight of carbon for the magnetic contact material, and 0.2 to1.0% by weight of carbon for non-magnetic contact material.

This invention relates to improvements in contact materials for use aselectrical contacts associated with switching devices commonly known asa vacuum switch or a vacuum type circuit interrupter and operative toswitch the associated contacts in a high vacuum to interrupt a flow ofcurrent.

Contact materials used in conjunction with any type of switches arenecessary to have the ability to resist to welding or sticking whatever.Particularly, the contact materials suitable for use as electricalcontacts for vacuum switches are required to be excellent in property ofresisting to welding because such contacts are apt to be very welded oneach other due to their surfaces remaining clean in the associatedvacuum.

It is well known that electrical contacts excellent in property ofresisting to welding can be generally made of metals, having highmelting temperatures, such as tungsten and the like. In the vacuumswitches, a phenomenon called as chopping may frequently take place.This phenomenon is described as being breaking of an electric areestablished across the contacts upon separating them from each other andbefore a magnitude of alternating current flowing through the associatedcircuit reaches its zero point. Since high melting point metals such astungsten and the like have generally the extremely low vapor pressuresthey have the high chopping current characteristics. Thus the use ofelectrical contacts made of these high melting point metals inconjunction with the vacuum switches is not desirable because a hightransient voltage can be induced in the associated circuit.

Also the contact metals can be produced by the socalled powdermetallurgical technique in which a mixture of high melting point metalsin the form of a powder is compacted and sintered into an alloyedstructure. The resulting contact metals, however, have been foundunsuitable for use as electrical contacts associated with the vacuumswitch. This is because the particles of the powdered metals have largeamounts of gaseous elements ice adsorbed on their surfaces and thereforethe compacted and sintered alloys can not provide the electricalcontacts including only the gaseous elements in sufiiciently smallamounts. It is essential that the contacts suitable for use in thevacuum switches be extremely low in contents of gaseous elements.

One type of the contact materials very low in contents of gaseouselements is described in copending U.S. application Ser. No. 406,874entitled Process of Refining Electric Contact Materials filed on Oct.27, 1964 by the same applicant et al. and assigned to the same assigneeas the present application. According to the cited specification a metalselected from the group consisting of nickel, cobalt, iron, molybdenum,tungsten and alloys thereof is first melted in a vacuum melting furnacein a vacuum of 10" mm. Hg or less. Then dissolved into the resultingmelt is at least 0.2% by weight of carbon and the melt is maintained inits melted state in such a vacuum for a period of time, for example, 30minutes to remove oxygen originally included in the metal from the meltas carbon monoxide. It is to be noted that the melt should be preventedfrom contacting any refractory oxide. Nitrogen and hydrogen which mightbe included in the metal is promoted in their removal as carbon monoxidethus formed escapes from the melt. Then the melt is allowed to be cooledto room temperature to be solidified.

The contact materials thus produced have the low cutting through currentcharacteristics such that, as compared with the conventional tungstencontacts having a chopping current of approximately 10 amperesnickelcarbon alloy contacts having a cutting through current ofapproximately 3 amperes or less.

Accordingly, it is an object of the invention to improve theanti-welding property of the contact materials disclosed in the abovementioned specification without any increase in cutting through currentcharacteristics thereof.

According to one aspect of the invention there is provided an electricalcontact material including from 1 to 10% by weight of at least one ofbismuth and tellurium and the balance being nickel and incidentalimpurities.

Carbon may be included in an amount of from 1 to 2.2% based upon theweight of the contact material.

According to another aspect of the invention there is provided anelectrical contact material including from 0.05 to 5% by weight of atleast one of bismuth and tellurium, from 25 to by weight of copper andthe balance being nickel and incidental impurities.

The invention will become more readily apparent from the followingdetailed description.

The invention is based upon the discovery that bismuth and tellurium canincrease the ability of contact materials to resist to welding orsticking.

As previously described, the contact material according to one aspect ofthe invention includes from 1 to 10% by weight of at least one ofbismuth and tellurium and the balance being nickel and small amounts ofincidental impurities. The contact material may preferably include from1 to 10% by weight of either bismuth or tellurium, from 1 to 2.2% byweight of carbon and the balance being nickel and small amounts ofincidental impurities.

Since the present contact materials are intended to be used as theelectrical contacts disposed within the vacuum switches it is essentialthat the starting material should have been preliminarily refined suchthat they will include a gaseous element or elements in a very smallamount or amounts. In this connection, it is to be noted that theelement nickel having the negligible contents of gaseous elements as thestarting material can be readily utilized by refining it according tothe refining process using carbon as previously outlined. On the otherhand, as the elements, bismuth and tellurium in their solid phase areextremely low in contents of solid solutions with gaseous elementscontained therein, slow solidification of the same from their moltenstate can readily cause the contents of gaseous elements negligible.

The resulting nickel-carbon alloy is melted in an atmosphere of an inertgas or a vacuum and bismuth or tellurium in an amount within the rangeas above specified is added to the melt. This addition of bismuth ortellurium reduces the solubility of carbon in the melt to precipitateand float the excess of carbon on the surface of the melt. After theremoval of the precipitate, the melt is allowed to be cooled into asolid. The resulting alloy may include from 1 to 2.2% by weight ofcarbon.

It has been found that the addition of either bismuth or tellurium inamount of 1% by weight or more is effective for a decrease in weldingforce. The addition of either bismuth or tellurium in an amountexceeding is not only ineffective for improvements in the anti-weldingproperty but also it renders brazing of the contacts difficult.

As previously described, the contact materials according to anotheraspect of the invention include from 0.05 to 5% by weight of at leastone of bismuth and tellurium, from to 50% by weight of copper and thebalance being nickel and small amounts of incidental impurities. Thecontact material may preferably include from 0.05 to 5% by weight ofeither bismuth or tellurium, from 25 to 50% by weight of copper, from0.2 to 1.0% by weight of carbon and the balance being nickel and smallamounts of incidental impurities. For the same reason as described inconjunction with bismuth and tellurium, addition of copper reduces thesolubility of carbon in the melt. Therefore the last-mentioned type ofthe present alloys is less in content of carbon than the first-mentionedtype thereof.

The last-mentioned type of the present contact alloys is non-magneticand has the improved anti-welding property. It has been found that thistype of contact alloys is superior to the first-mentioned type ofcontact alloys in that a less amount of bismuth or tellurium has beenadded to the alloy to improve effectively the anti-welding property, Inaddition it is characterized by the nonmagnetic property.

Copper is added to the alloys for the purpose of rendering the latternon-magnetic at the operating temperatures. Bismuth and tellurium servealso to render nickel non-magnetic. Therefore copper may be partiallyreplaced by bismuth or tellurium. The use of contacts made ofnon-magnetic materials makes it possible to drive magnetically an arcstruck upon interrupting a flow of current thereby to facilitateinterruption of a heavy current whereas the use of contacts made ofmagnetic materials will prevent an effective magnetic field from actingon such an arc.

It is considered desirable to use copper, silver or their base alloys asa material for contacts for use in magnetically driving an arc struckupon interrupting a flow of current. The element copper or silver isrelatively low in boiling point but greatly disadvantageous in that thechopping current is high due to its good thermal conductivity. To avoidthis disadvantage, there has been previously proposed the utilization ofcontact alloys including either copper or silver and a low melting pointelement in a relatively large amount such as a Cu-20% Bi alloy. The useof the contact alloys including a relatively large amount of a lowboiling point element (which has simultaneously a low melting point)leads to the brazing or soldering operation or the evacuating operationperformed with difficulty. On contrast, the present alloys have the lowchopping current characteristics Without any low boiling point elementadded thereto and are required only to have added thereto the addingelement in a small content as previously described, only for the purposeof improving their anti-welding property.

Some of contact alloys of the prior art type including copper or silverand having a suitable low boiling point element or elements addedthereto for the purpose of reducing the chopping current could exhibitthe increased anti-welding property as the secondary effect. Suchalloys, however, had the low boiling point element or elements in a farhigh content or contents as compared with the present alloys. It is tobe noted that in practicing the invention, all the low boiling elementsdo not consistently contribute to an increase in anti-welding propertyof the resulting contact alloys. For example, the addition of lead couldrather aid in welding the resulting contacts to each other after a lowcurrent has passed through them for a long period of time. Asillustrated hereinafter, the antimony is less effective for resisting towelding than bismuth and tellurium and cobalt-carbon-tellurium alloysare inferior to nickel-carbon-tellurium alloys in anti-welding property.

Numerous contact materials were produced according to the process aspreviously described and cut into cylinders having a diameter of 24 mm.and a height of 15 mm. One half of the cylinders was machined to beplaned at both ends while the other half of the cylinders was machinedto have a sprerical surface having a radius of curvature of 50 mm. ateach end. The flat ended cylinders were put in contact with the roundended cylinders under a contact pressure of 18 kg. and had a flow ofcurrent having a peak magnitude I as specified in kiloamperes in thefollowing Table I for a half cycle of alternating current having aperiod T as also specified in milliseconds in Table I. Then the weldingforce with which the cylinder pairs were stuck or welded to each otherwas measured in terms of a tensile strength in kilograms in the axialdirection of the cylinders or in the direction in which the currentflowed through the cylinders. The results of meas urements are listed inthe following Table I. For comparison purposes Table I also illustratesthe welding forces for contact alloys including cobalt and antimony.

TABLE L-MEASURED TENSILE STRENGTH IN KILOGRAMS OF STUCK CONTACTS CurrentHeat Ingredients in percent No. by weight I =2 ka., I =4 ka., I =6 ka.,I =10 ka.,

TI2=15 msec. Tl2=20 msec. Tl2=25 msec. T/2=40 msec.

1 Ni-2.3% C 117 57 153 481 C 30 9 22 19 1 15 26 119 1 29 63 110 1 10 461 7 7 69 1 7 11 33 1 1 10 21 1 1 6 40 1 1 1 15 1 1 1 1 1 1 57 1 25 35 481 1 10 167 N1-1.8% C-2.8% I 1 1 1 73 16"-.. N1-1.8% 03.6% Te 1 1 1 50TABLE I.--MEASURED TENSILE STRENGTH IN KILOGRAMS OF STUCK CONTACTSCurrent Heat Ingredients in percent No. by weight I =2 ka., I =4 km, I=6 ka., I,,=10 ka.,

Tl2=15 msec. Tl2=20 msec. Tl2=25 msec. Tl2=40 msee.

17- N i-1.6% C-10% Te 1 1 8 12 18 Ni-1.5% C-% Te 1 1 12 19 19 Ni-40%Ctr-0.5% C-0.005% BL... 1 37 72 149 20 N 140% Oil-0.5% C-0.01% Bl. 1 1237 22 21 Ni-% Orr-0.5% 00.05% Bi 1 1 1 4 22-- Ni-40% Cu-0.5% (3-0.1 a i1 1 1 112 23-- N 140% (Eu-0.5% C-0.9% Bi 1 1 1 1 24" N 140% Ctr-0.4%04.9% B 1 1 1 263 25.. Ni-40 0 Cu-0.3% C-10%;Bi. 1 1 1 1 26-- N i-40 0Cu-0.5% C-0.005% To--- 1 55 78 120 27 Ni-40% Cu-0.5% C-0.0l% Te 8 68 13328. Ni-40% Cir-0.5% 00.04% Te. 1 1 7 25 29 Ni-40% (Du-0.5% C-0.1% Te"-.-1 1 5 1 30.. Ni40% Cir-0.5% 00.9% Te. 1 1 1 1 31 i-40% (In-0.3% 04.8%Te. 1 1 1 2 32 Ni-40% (Eu-0.2% C-9% Te-.. 1 1 1 208 In Table I the heatnumbers 8, 9, 10, 15, 16, 17, 21, 22, 23, 24, 29, 30 and 31 designatethe contact alloy of the invention. From Table I it will be seen thatthe addition of bismuth or tellurium in a content of 1% or 0.5% withoutor with copper included respectively clearly reduced the welding force.Also Table I indicates that the addition of antimony is not so effectivefor improving the antiwelding property as the addition of bismuth ortellurium. Further the cobalt-carbon-tellurium alloy was inferior to thenickel-carbon-tellurium alloy in anti-welding property.

The invention also has another characteristic feature that the contactalloys prepared according to the same are relatively high in temperatureat which they begin to be melted in spite of their containing a lowboiling point element. On the bases of measured variations in specificheat of the present and control alloys, temperatures at which the alloyswill begin to be melted were calculated and listed in the followingTable 11.

TABLE IL-TEMPERATURE AT WHICH ALLOY BEGINS TO BE MELTED In general,vacuum devices are desirably heated to a temperature of at least 250 C.and preferably 500 C. or more upon sealing off them in order to removeany gas adsorbed on the surfaces of their components. To th1s end, anysuitable brazing material having a melting point higher than themagnitude just specified must be used to secure the electrical contactsto their supports within the vacuum devices. Thus the fact that thecontact alloys are high in temperature at which they begin to be meltedis greatly advantageous.

Tellurium is somewhat difficult in handling as compared with bismuthbecause of its higher vapor pressure but it is superior to the latter inview of the standpoint of the temperature at which the resulting alloysbegin to be melted as shown in Table II.

It will be understood that cobalt similar in properties to nickel may beused as a material for the contacts of vacuum switches after it hasdeoxidized with carbon. The binary alloys including cobalt and carbonare superior to the nickel-carbon alloys in that the former are less inwelding force. However, cobalt-carbon-bismuth alloys have thetemperatures at which they begin to be melted, less that the meltingpoint of the element bismuth or 269 C. Further, thecobalt-carbon-tellurium alloys have the temperatures as high as thenickel-carbon-tellurium alloys as shown by way of example in Table IIbut the cobalt base alloys are inferior to the nickel base alloys inantiwelding property as listed in Table 1. Therefore the substitution ofthe cobalt base alloy for the nickel base alloys has no advantage.

It is to be noted that the temperatures at which the present alloysbegin to be melted remain unchanged regardless of the amount of bismuthor tellurium added as long as the contents of ingredients involved arewithin the ranges as previously specified. When the particular alloy hasbeen heated to that temperature (at which it begins to be melted) allthe entire weight of the alloy is not immediately melted. The alloy hasits molten portion dependent upon both the degree of temperature bywhich the temperature of the heated alloy exceeds the melt-initiatingtemperature and the content of bismuth or tellurium and increased inweight as both factors in crease. Accordingly, with the content ofbismuth or tellurium low, the alloys are permitted to be actually heatedto temperatures greatly exceeding the respective melt-initiatingtemperatures.

Finally, as to the permissible amounts of incidental impuritiescontained in the present alloys, the elements particularly harmful forthe electrical contacts of the vacuum switches, for example, oxygen,nitrogen and hydrogen should be removed therefrom through sufiicientrefining of the starting materials in the manner as previously outlined.Other incidental impurities need not be removed from the startingmaterials. This allows the use of commercial grade starting materials inpracticing the invention, provided that the contents of the particularlyharmful gaseous impurities, such as oxygen, nitrogen, hydrogen, etc. arenegligible.

As previously described, the starting materials are preliminarilyrefined through the use of carbon. Thus the materials may include from0.02 to 2.5% by weight of carbon. It has been found that the presence ofcarbon in a content of from 0.02% to 2.5% as one of the ingredients orincidental impurities does not adversely afiect the anti-weldingproperty and the cutting through characteristics. If desired, carbon maybe removed from the starting materials.

While the invention has been described in terms of bismuth or telluriumit is to be understood that the present contact material may include thetotal amount of at least one of bismuth and tellurium with satisfactoryresults.

What I claim and desire to secure by Letters Patent is:

1. An electrical contact material for use with vacuum switches includingfrom 1 to 10% by weight of at least one metal of the group consisting ofbismuth and tellurium and the balance being nickel and incidentalimpurities.

2. An electrical contact material according to claim 1, furtherincluding from 1.0 to 2.2% by weight of carbon.

3. An electrical contact material according to claim 1, in which saidmetal is bismuth only.

4. An electrical contact material according to claim 3, furtherincluding from 1.0 to 2.2% by weight of carbon.

5. An electrical contact material according to claim 1, in which saidmetal is tellurium only.

6. An electrical contact material according to claim 5, furtherincluding from 1.0 to 2.2% by weight of carbon.

7. An electrical contact material for use with vacuum switches includingfrom 0.05 to 5% by weight of at least one metal of the group consistingof bismuth and tellurium, from 25 to 50% by weight of copper and thebalance being nickel and incidental impurities.

8. An electrical contact material according to claim 7, furtherincluding from 0.2 to 1.0% by weight of carbon.

9. An electrical contact material according to claim 7, in which saidmetal is bismuth only.

10. An electrical contact material according to claim 9, furtherincluding from 0.2 to 1.0% by weight of carbon.

11. An electrical contact material according to claim 7, in which saidmetal is tellurium only.

8 12. An electrical contact material according to claim 11, furtherincluding from 0.2 to 1.0% by weight of carbon.

References Cited UNITED STATES PATENTS 1,959,509 5/1934 Tour 75-1592,236,975 4/1941 Muller et a1. 75-159 2,309,103 1/1943 Crampton et al.75-159 2,867,550 1/1959 Weber 75170 RICHARD O. DEAN, Primary Examiner.

US. Cl. X.R.

